1. Field of the Invention
The invention relates to a digital transmission system comprising at least a transmitter transmitting a signal that includes an initialization sequence and at least a receiver which is synchronized with the transmitter by correlation means which recognize the initialization sequence.
2. Discussion of the Related Art
It is a current problem to synchronize a signal receiver with a signal transmitter each placed at either end of a transmission channel. The synchronization may be realized by detecting a sequence transmitted at the initialization of the transmission.
The document "Frame Synchronization Techniques" R. A. SCHOLTZ, IEEE Transactions on Communications, Vol. COM-28, no. 8, 1980, pages 1204-1213 describes a frame synchronization technique based upon a correlation between an initialization sequence and a reference sequence. When the two sequences are identical, there is maximum correlation and a correlation signal is transmitted which is used for establishing the synchronization. But, such a technique has limited possibilities when the transmission channel produces distortion.
This situation occurs in digital transmission systems, for example, telephone systems, for which there is a transceiver operating in the transmit mode at one end of a channel, whereas at the other end there is another transceiver operating in the receive mode, and vice versa. In such a bidirectional mode of operation it is necessary to synchronize the two transceivers with each other during an initialization period. To provide this synchronization, one uses synchronizing signals transmitted by the transmitter and identified by the receiver. To obviate these synchronization problems caused by transmnission channels, the synchronizing signals are initialization sequences. The synchronization of the receiver may be obtained by making use of the repetition of the signals transmitted by the transmitter.
It is necessary to identify precisely the moment at which the receiver has recognized the initialization sequence, because a poor precision results in synchronization defects and/or excessive complexity of the receiving material.